The present invention relates to a communication method applied to a spread-spectrum communication system.
A spread-spectrum communication system has been recently developed and put to the practical use, and has an advantage which is immunity to interference such as multipath interference and color noise as compared with the usual narrow-bandwidth communication.
The spread-spectrum communication system, however, involves a problem that the high-speed data transmissions need to use a bandwidth widened by a factor of a spread-coefficient of a signal to be transmitted. For example, when data of 1 Mbps is transmitted by using a code having a spread-coefficient of 11, the data transmission rate is 11 Mcps (chips per second) . If data of 10 Mbps is transmitted, the chip rate is 110 Mcps. This condition is difficult to realize since a necessary bandwidth is of 110 MHz to 220 MHz and the circuit must operate at an increased rate.
Accordingly, the present applicant has proposed two methods of increasing the data transmission rate without increasing the chip rate by applying multiplexing spread-spectrum signals. One of the methods is a spread-spectrum signal multiplexing system described in Japanese Laid-open Patent Publication (TOKKAI HEI) No. 9-55714 and the other mission system having a multiplex portion and a not-multiplexed portion, which have filed in Japanese Laid-open Patent Publication (TOKKAI HEI) No. 9-298491.
The use of these methods can increase the data transmission rate without increasing the chip rate.
The latter prior art method mentioned above use a data format having a not-multiplexed (simplex) portion and a multiplexed portion while the former having all multiplexed portions.
The data transmission format including the simplex portion and the multiplex portion is used for providing the compatibility of the data communication system. In this system, each station can select a desired multiplexing number for data transmission and, therefore, a receiving station can not receive data without previously knowing the multiplexing number of the transmission.
With a system using the previously determined multiplexing number, each station can transmit all signals in a multiplexed state.
With a system not changing a predetermined multiplexing number, a transmitting station must inform a receiving station of the number of data multiplexed and transmitted thereto.
For this purpose, a simplex sequence portion of the data transfer format is written with common information including the multiplex number which will be selected by the receiving station.
The data communication is usually conducted between a transmitting station and a receiving station when the latter selected the informed multiplex number thus decided at and transmitted from the former.
In multiplexed data transmission, the characteristics of a carrier-to-noise power ratio to an error rate become worse as the multiplex number increases. Correlatively improved characteristics were obtained by applying a technique proposed by the present applicant in Japanese Patent Application Serial No. 8-13963. At BER=10.sup.-5 (where BER is Bit Error Rate) a necessary C/N value of 5-multiplex transmission without correlation improvement differs by 15 dB from that of the simplex transmission. There is still a difference of about 7.5 dB between the 5-multiplex transmission with correlation improvement according to the method proposed by the present applicant and the simplex transmission.
An example of packet mode communication by the above-mentioned multiplex system is described as follows:
The communication is now conducted between Stations A and B. The station A transmits a packet of data to the station B. Upon receipt of the data packet the station B transmits an ACK (acknowledge) signal or a NAK (Negative acknowledge) signal to the station A if the received data was correct or incorrect. This judgment is usually made by checking such an error detection code as a CRC (Cyclic Redundancy Check) contained in the received data.
On receipt of the ACK signal, the station A transmits a next packet of data to the station B. With the NAK signal received or no response from the station B, the station A re-transmits the same packet once transmitted to the station B.
In a general data communication system, a transmission error may occur in a packet containing a long data portion but ACK and NAK signals containing short data may rarely be subject to transmission error.
However, a multiplex transmission system previously proposed by the present applicant encountered a problem that an increase of the multiplexing number is associated with a larger increase of the transmission error rate as compared with that in a typical conventional data communication system and even the ACK and NAK signals from the station B may also be lost or incorrectly received. With incorrectly received response signal, the station A must re-transmit the packet once transmitted to the station B even if the latter correctly received the same packet. If so, the station A transmits the unnecessary packet instead of a next packet, resulting in lowering total throughput of the communication.